CHARACTERIZATION AND MICROBIAL UTILIZATION OF EXUDATE MATERIAL FROM THE RHIZOSPHERE OF LOLIUM-PERENNE GROWN UNDER CO2 ENRICHMENT

The effects of elevated atmospheric CO2 concentration on alterations, both qualitatively and quantitatively, of exuded compounds from the ro ots of Lolium perenne seedlings were investigated by growing plants in a sterilised sand microcosm unit. In addition, the effect of CO2 trea tment on carbon substrate utilisation of microbial populations extract ed from the rhizosphere of L. perenne seedlings grown in soil microcos m units was examined and alterations on microbial activity and diversi ty assessed using a commercially-available redox-based sole C source u tilisation test (Biolog(R)) including additional exudate compounds. Bo th types of microcosm units (sand and soil) were maintained at specifi c growth conditions under two CO2 regimes (450 and 720 mu mol mol(-1)) . Growth of L. perenne seedlings from both types of microcosm units wa s enhanced under elevated atmospheric CO2 although the root-to-shoot r atios were not significantly altered, indicating no gross change in dr y matter partitioning. Cumulative total organic carbon (TOC) release i n the exudate material over the duration of the experiment was signifi cantly (P less than or equal to 0.05) higher from ambient-grown seedli ngs despite a significant (P less than or equal to 0.05) increase in t he dry weight of roots of the elevated CO2 grown seedlings as determin ed at harvest. Over the individual sampling periods TOC release was si gnificantly (P less than or equal to 0.05) higher from elevated CO2 gr own seedlings on only one occasion (21 d). Qualitative differences, me asured between d 1-6 and 14-18, also occurred with elevated CO2 treatm ent decreasing the amount of phenolic acids and total sugars at the la tter sampling period compared to ambient CO2 seedlings. Total numbers of bacteria were significantly (P less than or equal to 0.05) decrease d under elevated CO2 although culturable numbers significantly (P less than or equal to 0.05) increased. This increase in culturable microor ganisms may explain the faster carbon source utilisation rates of the elevated CO2 treatment. No change in morphotypes of microbial colonies were observed suggesting a quantitative difference due to elevated CO 2 treatment only. (C) 1998 Elsevier Science Ltd. All rights reserved.